Individual tunable laser sources are often used in optical systems as tunable swept laser sources. However, because it is common for the wavelength range of a single tunable laser to be insufficiently wide for the desired performance of the system, the output of multiple tunable laser sources are often combined to achieve the proper wavelength range for the application. In such a tunable swept multi-laser system, it is desirable to have only a single laser output electromagnetic radiation into the optical system at a given time.
In simple systems, the output of multiple laser sources is combined using an optical coupler and non-desired tunable laser sources in the system are simply turned off. In practice, turning off non-desired laser sources has the consequence of increasing scanning time and/or instability of the swept laser signal. As a laser is turned on in a system, the system can either wait for the laser output to stabilize by pausing data collection or acquire individual data sets for each tunable laser sweep and, in post processing, combine the separate datasets into one combined dataset. Both of these strategies require additional time resulting in a reduced sweep speed and a slower repetition rate.
In most swept applications, there are significant performance advantages, both for speed and stability, to leaving all the separate tunable laser sources on and separating the signal by another means. One such means is an optical switch, but cost, stability and switch time can all limit the performance of the system. In other systems, a wavelength division multiplexer (WDM) may be employed, but the isolation between the optical outputs is largely determined by the isolation of the WDM, which is limited and not sufficient for many applications. Another alternative is to power off the semiconductor optical amplifier. However, the laser continues to output light when the semiconductor optical amplifier is turned off, and the leakage power may contaminate the overall spectrum of the signal.